Most modern smartphones use scratch-resistant glass on their screens, but every once in awhile you'll see that your phone still gets some fine scratches. XDA Developers nails down the source: it's not the change or the keys in your pocket, it's the sand.

XDA does a bit of experimentation with a number of Spigen screen protectors, which use tempered glass to keep scratches off your phone (not unlike the "Gorilla Glass" that coats most modern smartphones). They test it against keys, coins, sand, and other materials to see what scratches it the most. Surprisingly, most materials didn't leave a mark, but that hard, gritty sand that's floating around in your pocket—or on your desk, or anywhere else—actually has the potential to leave some scratches in your phone, showing us that screen protectors can still be useful. Check out the video above to see their tests in action (though close your eyes at the end, it gets pretty scary when she does the drop tests).